The present invention relates to primer compositions and more specifically the present invention relates to primer compositions for room temperature vulcanizable silicone rubber compositions.
Primer compositions for adhering various materials to substrates is well known. The function of the primer composition is that when a composition does not adhere to a substrate by being placed thereon by mechanical reaction, there is applied a primer composition to the substrate and the composition is applied thereover such that the composition that is applied over the substrate has a stronger adherence to the substrate than was the case previously.
Primer compositions for room temperature vulcanizable silicone rubber compositions and heat vulcanizable silicone rubber compositions are well known. However, recently there has entered an innovation in the field--self-bonding room temperature vulcanizable silicone rubber compositions and self-bonding heat vulcanizable silicone rubber compositions have been developed. Briefly, such self-bonding compositions eliminate the need for a primer to adhere the composition to metal, masonry or plastic substrates by the insertion of a self-bonding ingredient in the uncured compositions.
It should be mentioned here as is probably well known to a worker skilled in the art that room temperature vulcanizable silicone rubber compositions comprise a two component system which when the two components are mixed and applied over a substrate will cure to a silicone elastomer. The one-component room temperature vulcanizable silicone rubber compositions are those compositions which are prepared to a single mixed component in an essentially anhydrous state and then the composition is applied to a substrate and in the presence of atmospheric moisture it cures to a silicone elastomer. The insertion of various additives such as, nitrogen functional silanes, as that disclosed in Lampe/Bessmer, U.S. Pat. No. 3,888,815, results in a self-bonding room temperature vulcanizable silicone rubber composition which will bond to various substrates without the need for a primer. The Lampe and Bessmer patent is hereby incorporated into the present case by reference.
In the same way, various types of one-component room temperature vulcanizable silicone rubber compositions have been devised which have self-bonding ingredients mixed into the composition such that the composition can be applied over a substrate such as, a masonry, concrete or a plastic substrate and adheres to the substrate with superior adherence when it is cured to a silicone elastomer in the absence of a primer. Such self-bonding compositions have also been devised for heat vulcanizable silicone rubber compositions which incorporate in them a silyl isocyanurate or a silyl maleate as a self-bonding additive such that the heat vulcanizable silicone rubber composition upon being heated at elevated temperatures to cure it, will bond to a masonry, plastic or metal substrate in the absence of a primer.
As a worker skilled in the art knows, a heat vulcanizable silicone rubber composition is one having a linear diorganopolysiloxane polymer gum having a viscosity of 1,000,000 to 200,000,000 centipoise at 25.degree. C., and which may have some vinyl unsaturation in it and which is cured in the presence of a peroxide catalyst to a silicone elastomer, which curing takes place at elevated temperatures, such as temperatures above 100.degree. C. The only difficulty with such self-bonding silicone compositions is that they are usually a very complex type of system and they require the addition or presence in the composition of very complex chemical self-bonding additives, which unduly increase the cost of the composition.
Another disadvantage is that in some cases such self-bonding silicone compositions, without the use of a primer, do not have as high an adherence to the substrate as would be desired; for instance, in the situation where the cured silicone elastomer is under water immersion for a substantial period of time.
Accordingly, in spite of such self-bonding silicone composition it is still desirable to have an effective primer composition for silicone compositions that are not self-bonding but through the use of said primer composition adhere the silicone composition to whatever substrate with a high level of adherence.
It is especially advantageous to have an effective primer composition for all different types of silicone compositions and more specifically for some of the more simplier formulated one-component room temperature vulcanizable silicone rubber compositions or two-component room temperature vulcanizable silicone rubber compositions which do not have self-bonding additives in them and which will at a minimum expense adhere them to a variety of substrates especially when the substrate and the cured silicone composition is to be subjected to a large amount of water immersion.
As a pertinent prior art of such primer composition reference is made to the U.S. patent of Bylsma entitled "Primer for Room Temperature Vulcanizing Polyurethanes", U.S. Pat. No. 3,453,136. It should be noted that the primer composition of this patent is disclosed to be effective for bonding room temperature vulcanizable polyurethane to substrates such as, glass, aluminum, stainless steel, concrete and marble. Nowhere does this patent disclose that its primer composition could be utilized for bonding room temperature vulcanizable silicone rubber compositions or even, for that matter, heat vulcanizable silicone rubber compositions. In addition, nowhere does the patent mention the bonding of room temperature vulcanizable polyurethanes to plastics. Although the patent specifies that the primer composition can be utilized to bond polyurethane to various substrates, the only substrates that are disclosed for such bonding are specifically, glass, aluminum, stainless steel, mild steel, concrete and marble. Nowhere does the foregoing Bylsma patent disclose that its primer composition is suitable for the bonding of polyurethane compositions or any other type of composition to plastic substrates. Accordingly, it was highly unexpected to discover that the Bylsma composition was highly effective for bonding silicone rubber compositions to various substrates and was specifically an excellent primer composition for bonding room temperature vulcanizable silicone rubber compositions to plastics and more specifically to polystyrene and that such bond had a great resistance to degradation even under substantial immersion under water at room temperature or at elevated temperatures. It should also be noted with respect to the Bylsma composition that while it was highly suitable for bonding room temperature vulcanizable silicone rubber compositions and heat vulcanizable silicone rubber compositions to plastic substrates, nevertheless, when utilized against metal substrates its adherency was not as much as would be desired.
Accordingly, it was highly desirable to improve the primer composition of the Bylsma patent such that the primer composition would function in a superior manner to bond silicone compositions and more specifically room temperature vulcanizable silicone rubber compositions not only to plastic substrates but also to metal.
Accordingly, it is one object of the present invention to provide for a method for bonding room temperature vulcanizable silicone rubber compositions to plastic substrates utilizing an acrylate functional primer composition.
It is another object of the present invention to provide for a method for bonding room temperature vulcanizable silicone rubber compositions to plastic substrates which primer composition produces a superior bond between the cured silicone composition and the plastic substrate even when the bond was exposed to a substantial amount of water immersion.
It is an additional object of the present invention to provide for an improved primer composition for room temperature vulcanizable silicone rubber compositions so as to bond such compositions with superior adhesion both to plastic and metal substrates.
It is still an additional object of the present invention to provide for an improved primer composition which will bond room temperature vulcanizable silicone rubber compositions with superior adhesion to metal and plastic substrates even under water immersion.
These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.